This invention relates to an iron core of an electromagnet and a method of producing the same, and more particularly to an iron core of an electromagnet suitable for use in accelerators, etc., composed of the stacked steel sheets and a method of producing the same.
Usually, an accelerator is equipped with a ring for accelerating or storing accelerated charged particle beams. The ring has an electromagnet for deflecting these charged particle beams. The electromagnet is composed of a coil and an iron core of the stacked steel sheets.
Conventionally, the steel sheets to be stacked are beforehand punched out into a specified shape or machined to provide a groove therein and the desired accuracy is obtained in this state. After that, the punched steel sheets are stacked by means of a stacking device. The stacked steel sheets are pressed and deformed, or the stack length difference is corrected. After all these steps, the sheets are welded or bonded together to form a one piece iron core.
Such an iron core will be described hereinunder by referring to an example of a sector iron core of an electromagnet.
The sector iron core is produced by stacking, in a direction of a deflection angle of charged particle beams or circumferentially, the required number of punched steel sheets each of which is shaped to provide a substantially C-shaped cross section and has a groove. Thereafter, the stacked sheets are integrally welded together by weld beads through end plates and side plates. The sectional shape of the sector iron core is symmetrical with respect to a horizontal axis. In the sector iron core, gaps are generated at an outer peripheral portion thereof because of the difference in the length of the outer and the inner arcs when steel sheets are stacked in the direction of deflection angle or circumferentially. Therefore, steel spacers are usually inserted in these gaps. Incidentally, the steel sheet has a thinner thickness at ends thereof depending on a capability of the rolling mill. These are thickness deviations called "edge drops". For this reason, especially in the case of an iron core of a wider section, gaps are generated on the ends of the iron core section when steel sheets are stacked in the direction of deflection angle of charged electron beams or circumferentially. Accordingly, steel sheet spacers are also inserted in the gaps in this part. Coils are disposed in a wider part of the groove and a beam duct is placed in a narrower part thereof for serving as the path for charged particle beams.
In the above-mentioned structure, however, gaps generated at the ends of the iron core caused by the thickness deviation cannot be completely eliminated even if these gaps are to be reduced by inserting steel sheet spacers. It is particularly impossible to completely fill with the steel spacers the gaps generated in an interior of a sector iron core of an electromagnet. This is because the shape of a gap is complicated depending on the producing conditions and so on, so that it is practically impossible to machine the steel spacer to adapt to the shape of the gap. Therefore, these gaps cannot be completely filled with the spacers and as a result an iron core of low density is produced.
Furthermore, in the conventional iron core, steel sheets are integrated into a one piece iron core after finishing all steps of processing (groove formation and general machining). Therefore, the deformation occurs in the iron core due to the welding heat on the occasion of integration. A groove which has been worked beforehand has low strength and this deformation is especially remarkable when heat is applied to such groove. The same occurs when gaps are existent.
Especially, when these iron cores used in an accelerator, a decrease in accuracy makes the path for charged particle beams unstable if the above-mentioned defect occurs in the iron core of electromagnet, because the bending electromagnet has the important function of guiding the path of charged particle beams that travel within the accelerator. Moreover, a decrease in the density of the iron core poses problems, such as lack of magnetic force (accuracy and size of a magnetic field generated by the iron core).